As far back as the Middle East oil crisis in the in 20th century, to save the energy, the high efficient condensing boiler was developed in Europe. The outstanding feature of the boiler is that the efficiency is 10% higher than the conventional boiler. Abundant water vapor in the flue is condensed and releases the latent heat of vaporization because the flue temperature can be decreased to below the dew point. This has the effect of energy saving. The condensing heat exchanger is developed and designed based on the principle of the condensing boiler.
The heat exchanger of the conventional (non-condensing) boiler is made by carbon steel or cast iron. The flue temperature is higher than 150° C. normally. The heat exchanger is not designed to absorb the sensible and latent heat when the water vapor condenses, and there is no condensate.
The condensing boiler is high efficient boiler with the features of energy saving and environmental protection. It is the future of the boiler industry and has been widely used. The life the condensing boiler will be shortened significantly if the carbon steel or cast iron is used because the boiler generates a lot of acid condensing water. So the material of the condensing heat exchanger should be stainless steel or cast aluminum. At present, most condensing heat exchangers are made with stainless steel tube or cast aluminum.
It is a proven technology to make cast aluminum heat exchanger, but the capacity is limited. It is very difficult to make the big cast aluminum parts. Typically, the bigger the cast, the higher the mold cost, the more complex manufacturing process and the higher scrap rate.
The efficiency can be around 96% maximally by using stainless steel or cast aluminum.
There will be no condensing water if the return water temperature is higher than 60° C. At this point, only the sensible heat in the flue can be saved. The heat efficiency of the non-condensing boiler is only around 87%.
The air pre-heater is applied in the large boiler in power stations normally. There is no such application in the heating boiler.
The conventional heat exchangers are designed according to the requirements from the different customers and the sizes of the heat exchangers are varied very much according to these requirements. Because there are a lot of components involved in the manufacture of heat exchangers, manufacturing many sizes of heat exchangers is not good practice for mass production.
While conventional heat exchangers have relatively good performance, there is still room for significant improvements in performance. A specific list of technical problems in need of improvement include: dead zones of flue flow and insufficient heat exchanging because of the poor heat exchanger structure design; increase the flue side heat transfer surface and efficiency by improving the heat transfer structure; to make the size smaller under the same heat transfer output; to integrate an air pre-heater into the heat exchanger to get the opportunity of third heat exchanging; increase the temperature of the inlet air; and decrease the flue temperature further. Accordingly, there is a need in the art to improve the heat exchanger.